In a phased array ultrasound imaging system, the ultrasound transducer includes an array of transducer elements. To support this array of transducer elements, the system includes a plurality of parallel channels, wherein each channel includes a transmitter and a receiver connected to one of the transducer elements in the array. Each transmitter outputs an ultrasound pulse through a transducer element into an object to be imaged, typically a human body. The transmitted ultrasound energy is steered and focused by applying appropriate delays to the pulses transmitted by each element in the array so that the transmitted energy arrives at a desired point in-phase, thus the energy adds constructively at that point. This causes a portion of the pulse to be reflected back to the transducer array by various structures and tissues in the body. As the pulse of ultrasound energy passes through the object to be imaged, a continuous reflection signal returns to the transducer array. The portions of the reflected signal received earliest by the transducer array are representative of those portions of the object closest to the transducer array. In general, the amount of elapsed time from when the pulse is transmitted until the signal is received by the transducer is representative of the distance from the transducer.
Steering and focusing of the received ultrasound energy is affected in similar manner. In a receive beamformer, the signal received from each of the transducers is processed and delayed, and then the signals from all of the transducer channels are summed in a signal summation element. The delay for each element is selected such that the reflected energy received by each transducer from the desired point is input into the summing element in phase (at the same time), thus creating a received beam that is focused at the desired point. The delays may be varied dynamically so that the transmitted beam can be scanned over a region of the body, and the signals generated by the beamformer can be processed to produce an image of the region.
Ideally, the delay means will not affect the signal in any way other than to delay it. The attenuation and the phase of the frequency components of the signal being delayed should not vary with the amount of delay selected; otherwise, the signal summation of the several channels will be unevenly weighted and will not produce the desired results. Also, the preservation of the signal should remain constant over a relatively broad frequency range so that shorter, wide-band ultrasound transmission pulses may be used.
In many prior art systems, the ultrasound signals remain in an analogue state until after signal summing element. In such systems, the delay means are usually limited to implementations such as fixed lengths of transmission line and all pass, constant group delay filters.
In other prior art systems, the ultrasound signals are digitized prior to being delayed and summed. In such systems, the means for creating the delays are necessarily digital. A common method of delaying the digitized ultrasound signal is to pass the digital samples through a series of hardware registers which are clocked at the sampling frequency f.sub.s. For delays equal to an integer number of digitization intervals, each digital sample may be stored in a digital data storage device such as a Random Access Memory (hereinafter referred to as RAM); then the digital samples to be summed are properly aligned when extracted from the RAM. With either the hardware register or the digital storage device delay methods, the amount of signal delay is limited to an integer number of sampling intervals .tau., where .tau. is typically equal to .lambda./(4c), .lambda. is the wavelength of the transmitted signal and c is the velocity of propagation of the transmitted signal. However, for precise beam steering, a smaller amount of delay for each channel is often required (typically as small as .lambda./32c!). Passing the digitized ultrasound signal through a digital filter can provide the desired sub-sample period delay, as long as the original signal has been properly sampled. A continuous, band-limited signal which has been properly sampled can be completely reconstructed in the continuous domain. For this reason, digital filters can exhibit group delays (or equivalently, time delays) on signals which are less than the sampling period. The coefficients of the digital filter can be dynamically modified so that a range of delays can be selected. A relatively high order digital filter with a corresponding large number of coefficients is necessary to achieve an amplitude and phase response with respect to frequency which is independent of the selected sub-sample period delay. Because of the large number of channels, (e.g., 64 to 128 typical), there is a practical need to simplify components within the channels. Lower order digital filters exist that preserve the phase of the signal frequency components of an amount independent of the sub-sample period delay, over a wide frequency range, but such filters attenuate the amplitude of the signals.
Accordingly, it is an object of this invention to provide an improved ultrasound signal delay means for processing received signals from an ultrasound transducer array.
It is another object of this invention to provide an improved ultrasound signal delay means for processing received signals from an ultrasound transducer array which applies an independent delay to each of a plurality of ultrasound signal channels.
It is yet another object of this invention to provide an improved ultrasound signal delay means for processing received signals from an ultrasound transducer array which applies an independent delay to each of a plurality of ultrasound signal channels, and each of the channel delay means incorporates a low order filter.
It is a further object of this invention to provide an improved ultrasound signal delay means for processing received signals from an ultrasound transducer array which applies an independent delay to each of a plurality of ultrasound signal channels, each of the channel delay means incorporates a low order filter, and any undesirable signal characteristics caused by the low order filters are compensated by a filter following the channel signal summation element.